The present invention relates generally to a method of concentrating iron ore and, more particularly, to a method of upgrading fine-grained earthy hematite iron ores.
In recent years, the percentage of steel being produced throughout the world using electric arc furnaces has increased to approximately 30%. However, unlike some other steel manufacturing systems, electric arc furnaces require relatively high grade ore in which the total iron concentration is greater than 68% with an oxide gangue concentration of less than 3%. Materials with oxide gangue concentrations much beyond this level produce an excessive volume of slag and therefore are uneconomically feasible as charge material.
Numerous relatively high-grade iron ores with iron concentrations in the range of 60 to 65% are of an earthy nature. These ores, which proliferate in Western Australia, the Middle East, and Africa, are typically intimately associated with extremely fine grained silica and alumina of feldspathic origin. Unfortunately, due to the fine grain structure associated with these ores, liberating the iron and achieving the oxide contaminant levels required for electric arc furnace charge material is nearly impossible and is generally cost prohibitive.
Several different processes have been developed for recovering iron concentrates from ore. U.S. Pat. No. 2,944,884 discloses a technique for producing high iron concentrates from low grade ores such as taconite. In the disclosed technique, the low grade ore is first crushed to minus 3/8 inch or finer. The crushed ore is then mixed with a reactive form of carbon such that the mixture contains at least 50% more carbon than the theoretical quantity needed for complete reduction. The mixture is then heated for a period of time between 18 and 21 hours at a temperature of about 870 to 1100.degree. C. so that the iron becomes fully reduced and carburized. The material is rapidly cooled so that the iron carbide particles lose their malleability. The charge is then ground and the carburized iron particles containing at least about 0.65% carbon are magnetically separated from the gangue particles.
U.S. Pat. No. 2,986,460 discloses a direct reduction process in which the iron ore is mixed with a carbonaceous reducing agent and then heated in a rotary kiln at temperatures of about 900.degree. C. The material is then cooled under controlled conditions in a non-oxidizing environment. The reduced iron is then separated from most of the gangue and compacted into briquettes.
In a more recent advance, U.S. Pat. No. 4,416,688 discloses a technique for reducing high phosphorus iron ore. In the disclosed technique sponge iron produced by selective solid state reduction is ground using a ball mill. The hammering action of the ball mill causes the formation of iron flakes of approximately 0.01 to 0.1 millimeters in size along with finely divided oxide gangue. Conventional concentration techniques are used to obtain iron flake powder concentrate. The carbon content of the sponge iron must be kept to a minimum, preferably below 0.10% and at least below 0.25%. Besides placing restrictions on the carbon content, the ability of the disclosed process to obtain high iron concentrates with low oxide contents is hampered by the size of the iron flakes formed by the ball mill. As a result, the efficiency and therefore the cost effectiveness of this approach is lower than desirable.
Therefore a cost effective technique for obtaining super concentrate materials with high iron concentrations and low oxide concentrations from relatively rich, fine-grained iron ores is needed.